The invention provides a battery condition estimating method and a power system permitting to estimate aging level or discharge ability of a battery in high precision by reducing transient changes of response voltages caused by repetitive discharges. A recovery time after a discharge is prolonged with a number of times of discharge and a relationship between the number of times of discharge and the recovery time is approximated by using an exponential function in the battery condition estimating method of the invention. A discharge pattern as shown in FIG. 1 is formed by using the recovery time obtained from this approximation and substantially constant response voltage is obtained by discharging the battery in accordance to this discharge pattern. Then, impedance during a metastable condition is calculated from the measured response voltage and the discharge current and the aging level or the discharge ability of the battery is determined from the impedance.